Non-impact optical printers are becoming increasingly popular for producing texts and graphics, particularly in gray scale applications. In gray scale printing, multiple gradations and shades of gray can be printed in addition to black-and-white and "write white" or white-on-black printing. Gray-scale printing is typically used to create pictures rather than text.
In xerographic printers, an electrostatic charge is formed on the surface of a moving drum or belt. Selected areas of the drum or belt are discharged by exposure to light, e.g. from light emitting diodes (LEDs) or lasers. A printing toner is applied to the drum and adheres to the areas which have an electrostatic charge. The printing toner does not adhere to the discharged areas on the drum. The toner is then transferred from the areas on the drum having the electrostatic charge to a sheet of plain paper and is heat fused to the paper using well known methods. Characters are constructed in the well-known dot matrix fashion, with each character comprising a number of illuminated dots. Optical character generation devices are well known and are described in previous U.S. patents (e.g. U.S. Pat. No. 4,596,995).
In gray-scale printers, the shade of gray is determined by the amount of the electrostatic charge on the photo-receptive surface. Such printers may be used for reproducing photographs. For example, one type of optical printer uses arrays of light emitting diodes (LEDs) as the light source which exposes the photoreceptor surfaces. To create high quality images with an LED printer, each of the individual LEDs should produce the same amount of light output when they are activated by a specified signal. It is particularly important that each of the LEDs produce a uniform light output when the printer is a gray-scale printer. As the light output from each LED tends to vary significantly, a number of systems have been proposed to correct the variance in light output.
The amount of time that each LED has been on (its age) is one of two important time dependent factors which affect the amount of light output of each LED. It is believed that the rate at which the LED light output is reduced due to repeated use is unique to the particular LED. It is believed that the degradation rate is related to the number of defects or dislocations at the junctions in the crystalline lattice structure of the doped semiconductor of which LEDs are comprised as well as being related to the amount of strain in the material. Therefore, the degradation rate due repeated use or aging for an LED printhead is non-uniform between individual LEDs.
The other important time-dependent factor in the light output of the LED is the temperature of the junction. The light output decreases as the temperature increases, and by measuring the amount of decrease in light output due to temperature during manufacture and measuring the temperature during the operation of the printhead, the light output can be predicted. Compensation for the amount of degradation in light output due to temperature can therefore be accomplished. Since the amount of degradation due to temperature does not vary substantially from pixel-to-pixel, the compensation may be either performed by pulse-width modulation or by modifying the current at which the LEDs operate. With either technique, modifying the current or pulse-width modulation, it is unnecessary to measure the light output during operation, which increases the cost.
Prior art systems, such as U.S. Pat. No. 4,780,731 (Creutzmann), continuously monitor temperature and the LED light output and correct the light output at regular intervals based on the measured data. Such systems are expensive and a need has developed for a more cost effective system which does not sacrifice accuracy.